[Python] Recommended ways to split some functionality into functions, modules and packages?

There comes a point where, in a relatively large sized project, one need to think about splitting the functionality into various functions, and then various modules, and then various packages. Sometimes across different source distributions (eg: extracting a common utility, such as optparser, into a separate project).

The question - how does one decide the parts to put in the same module, and the parts to put in a separate module? Same question for packages.

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